The overall goal of the current proposal is to elucidate the intracellular cardiac myocyte signaling mechanisms by which anthrax lethal toxin induces acute heart failure. Although anthrax lethal toxin (LT) has been studied for half a century, the mechanisms by which this toxin produces cardiovascular collapse are poorly understood. Using a male Sprague Dawley rat model, we have reported that LT produces acute systolic heart failure, characterized by reduced ejection fraction, delayed ventricular filling and decreased contractile performance. Biochemical analysis of cardiac tissues revealed that acute LT treatment (2-4 hr) caused dysregulation of mitogen-activated protein kinase (MAPK) signaling, decreased myocardial phospholamban and troponin I phosphorylation, and increased enzymatic activity of protein phosphatase PP2A. Protein kinase A activity (phosphorylation) was also increased, suggesting a compensatory role for 21-adrenergic stimulation in mediating contractile function. The correlation between in vivo cardiac function and biochemical changes in left ventricular tissue suggests that cleavage of MEKs by anthrax LT causes an imbalance in ERK, JNK and p38 MAPK activity that results in PP2A activation and reduced contractile function. The objectives of this work will be to use a combination of pharmacologic and molecular approaches in primary cultures of rat cardiac myocytes and in vivo studies to (1) uncouple the direct and indirect effects of anthrax LT on cardiac contractility and calcium mobilization, (2) determine how crosstalk between the MAPKs ERK, p38 and JNK regulates PP2A activity, and (3) use in vivo and ex vivo models to distinguish between direct and indirect effects of LT on myocardial function and to verify in vitro findings. A better understanding of these signaling networks will provide valuable information related to the effects of anthrax LT on cardiac myocytes, as well as reveal critical intracellular signaling mechanisms that govern contractile function. This information could lead to rationale drug therapies that preserve cardiac function in active duty military/Veterans exposed to this biological agent, as well as individuals with other causes of acute heart failure.